PDF spam, the nuisance that flooded inboxes in early August and then quickly disappeared, is back and worse than ever.

According to multiple threat researchers at security vendors, tens of thousands of spam messages were blasted out last week with attached PDF files, which infect the recipients' PCs when viewed. The subject lines of the new crop of PDF spam are finance-related, according to security vendors, using phrases designed to get the recipient's attention such as "your credit report." These e-mails contain no text, simply the attachment.

"When opened, the PDF file uses the CVE-2007-5020 vulnerability via Acrobat Reader and [Internet Explorer 7.0] and downloads further malware from a server in Malaysia," according to security vendor F-Secure's recent blog post. "The target of the malware seems to be to create a botnet of infected machines to be used for further malicious activity."

This year's PDF spam raised eyebrows because of the sheer volume of the messages and the creative thinking on the part of spammers who figured out these messages could circumvent most antispam filters because they're not trained to read PDF attachments.

Because of the malware now hidden in the attachment, this round of PDF spam is significantly more malicious than August's blasts that were typically pump-and-dump stock messages.

The PDFs attached to those messages hid no malware, but attempted to persuade recipients to buy penny stocks in a little-known company so the stock price would be driven up and the spammer could sell at a profit.

While PDF spam disappeared by September as quickly as it arrived -- it went from 30 percent of all spam sent on August 7 to less than 1 percent on August 29 -- few security professionals are expressing surprise at its return.

Spammers will "exploit any vulnerabilities they can, which in Windows is about a quadrillion different places," says John Levine [stet], president of consulting firm Taughannock Networks and co-chair of the Internet Engineering Task Force's Anti-Spam Research Group, adding that he believes this PDF spam blast to be the latest incarnation of the Storm malware. "Using Acrobat has the added advantage that it works regardless of what mail program you use, so even people who use Eudora or Thunderbird could get bitten."

Adobe on October 22 released a security update for versions 8.1 and earlier of Adobe Reader and Acrobat.